It is a common practice with electrophotography to form a latent image on an image carrier and then develop the latent image with a developing liquid consisting of a carrier or solvent and toner, or fine colored particles, disposed in the carrier. The resulting toner image is transferred from the image carrier to a paper or similar recording medium and then fixed on the paper. When an image fixing device applicable to this type of process uses a volatile carrier, a fixing method using heat is predominant over the other fixing methods. Conventional fixing methods using heat are generally classified into two types, i.e., a contact type fixing method causing a roller or a belt heated to a preselected temperature to contact the toner image and a noncontact type fixing method heating the toner image by the radiation of, e.g., infrared rays. As for heat transfer efficiency, the contact type fixing method is far more desirable than the noncontact type fixing method.
In the image fixing device using the contact type fixing method, even when the carrier is left in the toner image, it is partly or almost entirely evaporated by heat during fixation. It is therefore possible to increase coupling forces acting between the toner particles and between the toner particles and the recording medium by pressure. As a result, the toner image can be desirably fixed on the recording medium.
However, the problem with the volatile carrier is that an arrangement for handling the vaporized carrier is required. This limits the applicable range of the fixing device of the type described. A current trend in the imaging art is, therefore, toward the use of a nonvolatile carrier containing no volatile components. A nonvolatile carrier, however, brings about another problem that it exists between the toner particles forming the toner image and between the toner particles and the paper even when the particles are softened or melted by heat. As a result, the carrier obstructs the cohesion of the toner particles and the adhesion of the particles to the paper at the time of pressing, i.e., the fixation of the toner image. Moreover, transfer of the toner to a heat roller or contact member, i.e., so-called offset, occurs. In addition, the toner image carried on the paper is disturbed.
To solve the above problems, Japanese Patent Laid-Open Publication No. 9-281753, for example, proposes an image fixing device using a Johnson-Rahbec effect. The device taught in this document fixes a toner image formed on a paper or similar recording medium by use of a developing liquid consisting of a carrier and toner dispersed in the carrier. Specifically, the device includes a conductive roller or electrode and another conductive roller capable of respectively contacting the front or image surface and the rear of a paper. A current is caused to flow between the two conductive rollers in the direction of thickness of the toner image, so that the toner particles firmly cohere together and firmly adhere to the paper (preliminary fixation hereinafter). After the preliminary fixation, heat and pressure are applied to the paper between a heat roller and a press roller for thereby fixing the toner image on the paper (actual fixation hereinafter). However, this kind of device is not desirable because the preliminary fixation and actual fixation each using exclusive rollers lower the fixing efficiency and obstruct the compact configuration of the device.
Assume that to implement both of the preliminary fixation and actual fixation with a single heat roller and a single press roller, the heat roller is provided with resistance low enough to cause a current to flow between the heat roller and the press roller in the direction of thickness of the toner image. Then, a crash or similar defect existing in the heat roller would cause abnormal discharge to occur at the position where the heat roller and press roller contact. The resulting concentration of a current would destruct the heat roller and obstruct expected fixation.
Another problem with the conventional fixing device using the heat roller is that as the surface temperature required of the heat roller rises relative to room temperature, a period of time necessary for the surface temperature to reach the required surface temperature, i.e., a warm-up time, increases. In addition, power consumption is aggravated. Moreover, the heat roller with such a high surface temperature causes the paper to crease or curl due to a decrease in water content and causes the electrical resistivity of the paper to drop, thereby varying image transfer conditions. For example, a full-color mode operation includes steps of forming a first image on a paper, fixing the first image, and forming a second image above the first image. In this case, if the electrical resistivity of the paper is lowered by the formation of the first image, a transfer bias must be increased at the time of formation of the second image; in the worst case, an adequate transfer bias for the second image is not available, rendering the resulting full-color image defective.
It has been customary with the image fixing device using heat to form one or both of the heat roller and press roller by using rubber, sponge or similar elastic material. The press roller and heat roller form a nip therebetween and fix a toner image formed on a paper or similar recording medium while conveying the paper through the nip. To form a nip broad enough to promote fixation, the device is constructed such that heat roller and press roller exert a great pressing force or such that a either the heat roller or the press roller has low hardness. The pressing force may be increased by increasing the mechanical strength of the two rollers, i.e., the volume of the core of each roller. However, an increase in the strength of the heat roller results in an increase in the heat capacity of the heat roller and therefore an increase in warm-up time. On the other hand, should the hardness of the heat roller or that of the press roller formed of an elastic material be reduced, the roller would suffer from permanent strain and decrease in durability.
In light of the above, use may be made of a heat belt or a heat film for fixing a toner image on a paper or similar recording medium. The heat belt and heat film each can form a nip width great enough to insure stable fixation without resorting to a great pressing force. Further, by adequately devising heat transfer between the belt or the film heated and the paper, it is possible to reduce offset.
However, the above heat belt or heat film scheme sophisticates the construction of the fixing device, compared to the heat roller and press roller scheme. Moreover, the heat belt is apt to be displaced to either side. In addition, the sophisticated construction and the displacement of the heat belt lower stability and make the device not feasible for high-speed applications.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Nos. 2,781,390 and 2,606,843.